Embryology of GI Tract

الرجاء دراستها مباشرة من الساليدات ,,, لكنها مطلوبة , في المجموعة التانية قرأت قراءة سريعة 33-15الساليدات من

غير ذلك فالشيت تغطي كل حرف هو بالساليد ضمن ترتيب يسهل و يبسط الحفظ

1-Development of Oral cavity

**the mouth has 2 sources of development:

(1) depression in the stomodeum (ectoderm)>>> floor of oral cavity

(2) cephalic end of the foregut (endoderm)

**Buccopharyngeal membrane (3) : separates the two points ( (1) + (2) )… disappears during the

3rd week so connection will occur between the oral cavity (1) and the foregut (2)

( جاء لتسهيل الحفظ و ربطه بأسبوع اضمحالله 3ترقيمه برقم (

** An imaginary line (at the site of this membrane) is created, separating the ectodermic

origins (anterior to the line) from the endodermic origins (posterior to the line)

>>> This line extends to:

- body of sphenoid (posterior!)

- Soft palate (middle)

-Inner surface of the mandible, inferior to the incisor teeth (ant.)

>>> Ectodermic origins (Ant. To the line ) >>>endodermic origins (Posterior to the line )

- Hard palate -Tongue

-Sides of the mouth - soft palate

- Lips - palatoglossus & palatopharngyeal folds

-Enamel of the teeth -floor of mouth

(1) (2)

(3)

2- Development of the salivary glands

** During the 7th week , a solid outgrowth of cells arises from the walls of the developing mouth (mucosa )

to invaginate and grow into the underlying mesenchyme C.T (1) that condense to form:

-the capsule of gland

- septa ( + blood vessels & lymphatic )>>> divide the gland into different lobes and lobules

>>>The Capsule and septa >> mesoderm

** The epithelial buds will go through repeated branching to form solid ducts (2) ( canalization )

** the ends of these ducts >>> form the secretory acini (3)

>>> both ducts and acinis go through canalization

>>> the difference between Endocrine and Exocrine ducts , that the endocrine ducts disappear during

development while the exocrine ducts will stay

** Ducts and acini of

- Parotid >>> Ectoderm

- Sub ( mandibular & lingual ) >>> Endoderm

(2)

(3)

(1)

2- Development of the Tongue

**floor of pharynx consist of 4 arches on each side, each arch has 3 types of tissue; (ectoderm, mesoderm & endoderm)

اختصارا للوقت , لو تكرمتم تدرسوا المعلومات ادناه مزامنة لربطها مع الصور المرفقة

Anterior 2/3

***Two lateral lingual swellings and one medial swelling, the tuberculum impar >>>originate from the first pharyngeal arch (at 4th week)

>>as the lateral lingual swellings increase in size, they overgrow the tuberculum impar and merge, forming the anterior two-thirds, or body, of the tongue.

>> Since the mucosa covering the body of the tongue originates from the first pharyngeal arch, sensory innervations to this area is by the mandibular

branch of the trigeminal nerve (lingual branch )

>> Special sensory innervations (taste) to the anterior two thirds of the tongue is provided by the chorda tympani branch of the facial nerve

(2)

Posterior 1/3 ***A second median swelling, the copula, or hypobranchial eminence, is formed by mesoderm of the second, third, and part of the fourth arch.

Forming the posterior part or root of tongue.

>>>sensory innervations to this part is supply by the glossopharyngeal nerve

indicates that tissue of the third arch overgrows that of the second. That's why we considered circumvallates papillae ( terminal sulcus ,V-shaped groove that

separate the body of the tongue from the posterior part ) as parts of posterior 1/3(embryological ) although they are in the anterior 2/3 (anatomically)

Epiglottis and the extreme post. Part of tongue *** a third median swelling, formed by the posterior part of the fourth arch, marks development of the epiglottis.

>>>Immediately behind this swelling is the laryngeal orifice, which is flanked by the arytenoids swellings.

>>> innervated by the superior laryngeal nerve, reflecting their development from the fourth arch.

Muscles of tongue

*** some of the tongue muscles probably differentiate in situ, but most are derived from myoblasts originating in occipital somites

>>>is innervated by hypoglossal nerve.

2- Development of the pharynx

**the floor of the pharynx, consist of 3 types of tissue: ( ecto , endo and mesoderm )

**The pharynx develops in the neck from the endoderm of the foregut.

>>>The endoderm is separate from the surface ectoderm by mesenchyme , which splits up on

each side to 5-6 arches , each arch form a swelling on the surface of the wall of the foregut.

>>> the result of these swellings :

- a series of clefts between the arches on the Ectodermal side … pharyngeal clefts

- Similar grooves are found on the lateral walls of the foregut , on the endodermal side

…..Pharyngeal pouches

>>>the foregut on this level is known as the pharynx

(2)

Nerve supply :

ventral rami of spinal nerves

( thoracic )

2- Development of the Ant.Abd.wall

**Following the segmentation of the mesoderm (each segment has its own innervations)

>> > the lateral plate mesoderm divides into:

- somatopleuric mesoderm >>> parietal that form serous membrane that line the

peritoneal, pleura & pericardial cavities

@from where the muscles of ant. Abdominal wall is derived

> skin and other layers that lie in front of the

ant.abd.muscles are derived from ectoderm

@ 3 layers are formed :

1) Ext. oblique

2) Int.oblique

3) Trans. Abdominus

these muscles form aponeurosis >> that form rectus sheath

-Splanchopleuric mesoderm>>> visceral mesoderm form serous membrane that line each organ

## both layers are lined by endo and ectoderm

>>>rectus abdominus muscle retains the indications of the segmental origin (the presence of tendinous intersections) >>> myotomes

>>>abd. Wall right and left sides of mesenchyme fuses together at 3 months into the midline to form the linea alpa. Before that , they make anterior

and posterior wall around rectus abdominus muscle > rectus sheath… lina alpa and rectus muscle lies within the sheath

2- Development of the umbilicus and the umbilical cord

**The amnion and the chorion fuse together to form amniotic cavity around the embryo , The amnion encloses the body stalk and the yolk sac with

their blood vessels to form the tubular umbilical cord

>>The mesenchyme core of the cord (whartons jelly) form a loose connective tissue which embed the following:

-Remains of yolk sac

-Vittelline duct (vittellointestinal ) , connects the umbilicus to the midgut (intestine ).if not obstructed >> Meckel's diverticulum

-Remains of allantois

-Umbilical blood vessels >>> 2 arteries that carries deoxygenated blood from the fetus to the chorion (placenta)

>>>2 veins carry oxygenated blood from the placenta, but the right vein will soon disappear

**Vitelline Duct Abnormalities

- a small portion of the vitelline duct persists on the antimesenteric border of ileum >>> forming an outpocketing , Meckel’s diverticulum or ileal

diverticulum .

- in 2-4% of people , 2 feet (40-60 cm ) from the ileocecal valve , 2 inches in length , 2 types of tissue > heterotopic pancreatic tissue and gastric mucosa

- may cause ulceration , bleeding or perforation , symptoms at this level suspected to be appendicitis

- Sometimes both ends of the vitelline duct transform into fibrous cords, and the middle portion forms a large cyst, an

enterocystoma, or vitelline cyst …..> portions of small intestine may protrude out through the umbilicus if not obstructed

2- Development of the lung buds

**the respiratory diverticulum (lung bud) appears as an outgrowth from the ventral wall of the foregut (pharynx ) at the 4th week .

** the location & the time of lung bud appearance are determined by signals from the surrounding

mesenchyme > heredity box or gene box , these signals including fibroblasts growth factors (FGFs)>

instruct endoderm .

**epithelium of the internal lining ( larynx, trachea, and bronchi, as lungs) , >> endodermal origin

**The cartilaginous, muscular, and connective tissue components of the trachea and lungs >> derived

from splanchnic mesoderm surrounding the foregut.

**skin & other layers >> ectodermal origin

>>>Initially the lung bud is in open communication with the foregut (A)

>>>diverticulum expands caudally,two longitudinal ridges, the tracheoesophageal ridges>> separate

it from the foregut (A+B) ,,,, fuse to form the tracheoesophageal septum, the foregut is divided into a dorsal portion, the

esophagus, and a ventral portion, the trachea and lung buds (C)

**respiratory primordium maintains its communication with the pharynx through the laryngeal orifice

Quick summary

* Mouth ,, 2 sources : stomodeum (ectoderm ) separated by buccopharyngeal membrane / rupture at 3rd

week

cephalic end of foregut ( endoderm )

,, cells arises from mucosa at 7th

month >> grow into mesenchyme , forming ducts by canalization + acini at the end * Salivary gland

- the capsule and septa >> mesoderm in origin

-parotid >> ectoderm

- submandibular and sublingual >> endoderm

*Pharynx ,, >>> endodermal origin

pharyngeal clefts on the ectodermal side

pharyngeal pouches on the endodermal side

*Tongue ,,, at 4th

week

Sensory innervations /motor Derived from

Mandibular branch of trigeminal taste >> chorda tympani

1st

pharyngeal arch ( 2 lat.lingual swelling + 1st

median ) Ant 2/3

Glossopharyngeal nerve Mesoderm of 2nd

,3rd

& part of 4th

arch ( 2nd

median swelling , copula )

Posterior 1/3

Superior laryngeal nerve 4th

arch ( 3rd

median swelling ) Epiglottis

Hypoglossal nerve / motor Myoblasts of occibital somites Tongue muscles

Posterior to the line Endoderm

Imaginary line Anterior to the line Ectoderm

Soft palate Body of sphenoid Hard palate

Tongue Soft palate Sides of the mouth

Palato ( glossus + pharyngeal ) folds

Inner surface of mandible lips

Floor of mouth Enamel of the teeth

http://www.youtube.com/watch?v=VG_DJqEB1_8*

http://www.youtube.com/watch?v=s2cNCUL1r3A

(الرجاء مشاهدتها حتى تترسخ االفكار و المعلومات

Esophagus

** At first is short then lengthens rapidly with descent of the heart and lungs

** The muscular coat >> (derived from splanchnich mesenchyme ) 1- striated muscles in the upper 2/3 ,, innervated by vagus

2- smooth muscles in lower 1/3 ,, innervated by splanchnic plexus

**Esophageal Abnormalities :

1) Esophageal atresia ( blind sac )

prevents normal passage of amniotic fluid into the intestinal tract, resulting in accumulation of excess fluid in the amniotic sac

(polyhydramnios)>>> congenital abnormalities in respiratory tract .

stomach may filled with air (air passes from trachea toward stomach )

projectile vomiting , during milk sucking

2) tracheoesophageal fistula >>> result of 1-spontaneous posterior deviation of the tracheoesophageal septum.

2-mechanical factor pushing the dorsal wall of the foregut anteriorly

- most common form > proximal part of the esophagus ends as a blind sac ( atresia )

, and the distal part is connected to the trachea by a narrow canal ( fistula ) just

above the bifurcation (A)

- Other types of defects in this region occur much less frequently ~ 1-2%

-(C) is easily solved by surgical interfere

3) esophageal stenosis : narrowing of the lumen ,, usually at lower 1/3

caused by : 1-incomplete recanalization

2-vascular abnormalities

3-accidents that compromise blood flow

4) congenital hiatal hernia : esophagus fails to lengthen sufficiently and the

stomach is pulled up into the esophageal hiatus through the diaphragm

Development of the glands

** derived from epithelium cells that project into underlying C.T >>> 1- exocrine ; maintain contact with surface via ducts

2- endocrine ; lose the direct contact with surface , duct degenerate during

development >>1. Cords

2. Follicles

Development of Stomach

>>> 4th week : - fusiform dilation of foregut

- The cephalic and caudal ends of the stomach originally lie in the midline

- attached to the dorsal body wall by the dorsal mesogastrium and to the

ventral body wall by the ventral mesogastrium.

>>> following weeks :

appearance and position change greatly as a result of >>>different rates of growth in various regions of its wall

>>>changes in position of surrounding organs

## positional changes : explained by assuming that it rotates around a longitudinal and an anteroposterior axis

rotation and disproportionate growth alter the position of these mesenteries.

1) around longitudinal axis ; rotates 90◦ clockwise causing

- its left side & left vagus nerve to face anteriorly

- its right side & right vagus nerve to face posteriorly >> grows faster than the anterior portion, forming the

greater and lesser curvatures

- pulls the dorsal mesogastrium >> left , creating a space behind the stomach called

the omental bursa ( lesser sac

-pulls the ventral mesogastrium to the right.

2) around anterioposterior axis , causing :

- caudal or pyloric part moves to the right and upward

- cephalic or cardiac portion moves to the left and slightly downward

: الرجاء االطالع على الفيديو في اللينك ادناه , لمزيد من الفهم

http://www.youtube.com/watch?v=AscKR_cQExY

hence , at final

position, its axis

running from above

left to below right.

>>> the fifth week :

* spleen primordium >> appears as a mesodermal proliferation between the two leaves of the dorsal mesogastrium

* With continued rotation , dorsal mesogastrium lengthens >> portion between spleen and dorsal midline swings to the left and fuses with the

peritoneum of the posterior abdominal wall >> spleen becomes intraperitonial ,,,, then connected to the body wall in the region of the left

kidney by the lienorenal ligament and to the stomach by the gastrolienal ligament

The posterior leaf of the dorsal mesogastrium and the peritoneum along this line of fusion degenerate *

*Lengthening and fusion of the dorsal mesogastrium to the posterior body wall also determine >

final position of the pancreas

>>> Abnormalities

1) Pyloric stenosis : when the circular and, to a lesser degree, the longitudinal musculature

of the stomach in the region of the pylorus hypertrophies

: most common in infants, believed to develop during fetal life.(3-6) weeks

>>> 3rd week :

- intraembryonic mesoderm on each side of the midline differentiates into (paraxial , intermediate and a lateral plate )

- When intercellular clefts appear in the lateral mesoderm, the plates are divided into two layers:

the somatic mesoderm layer and the splanchnic mesoderm layer

- The latter is continuous with mesoderm of the wall of the yolk sac

- The space bordered by these layers forms the intraembryonic cavity (body cavity ) … At first the right and left

sides of the cavity are in open connection with the extraembryonic cavity, but when the body of the embryo

folds cephalocaudally and laterally, this connection is lost

-- The peritoneal cavity is derived from the intraembryonic coleam caudal to the septum transversum